This invention relates to the manufacture of containers made of metal. In particular, the invention concerns a method of making metal containers using a deep drawing and ironing process.
It is well known to manufacture metal containers by deep drawing and ironing a metal blank. According to the known methods, a flat circular blank is stamped from sheet material by means of a blanking press. The blank, after application of a suitable lubricating composition, is then formed into a cup-shape by pushing the blank through a series of die rings by means of a punch. The cups produced by this first drawing and ironing step (otherwise known as "cupping") are work hardened and therefore subjected to an annealing treatment to render the material suitable for further forming before a second drawing and ironing step is performed on the cups by punching them through a second set of ring dies to reshape the cups to the required diameter and wall thickness.
The known method has proved highly successful for some metals such as aluminium and mild steel which have the required ductility, and containers made in this way are satisfactory for many applications, such as cylinders for containing gases at relatively low pressures. In other cases, however, and in particular in the case of high pressure gas cylinders, e.g. for carbon dioxide, it has been the practice hitherto to manufacture the containers from tube stock since safety requirements or other considerations demand that higher strength material, i.e. high strength steel, should be used, otherwise a much greater wall thickness is necessary to achieve the strength needed. The use of tube stock has the drawback that an end of the tube must be closed off. Defects can occur at the closed end and result in leakage of gas from the containers. This method is also labour intensive and costly in comparison with the deep drawing and ironing techniques as mentioned above. Attempts to extend the known deep drawing and ironing method to blanks of high strength steel proved unsuccessful. During the cupping process there is a stress concentration at the blank edge which results in the cups being flawed by cracks and fractures extending downwardly from the cup rims. Cups with such flaws are useless and are incapable of producing satisfactory containers. While lower strength steels could be used for high pressure gas cylinder manufacture, additional material required in order to obtain the required strength would negate the gain in economy from employing a deep drawing and ironing process.